On Tue, Nov 3, 2009 at 2:46 PM, Steven D'Aprano <st...@pearwood.info> wrote: > On Tue, 3 Nov 2009 09:19:38 am Greg Ewing wrote: >> Cameron Simpson wrote: >> > Personally, I'm for the iteration spec in a lot of ways. >> > >> > Firstly, a .get()/.pick() that always returns the same element >> > feels horrible. Is there anyone here who _likes_ it? >> >> It doesn't sound very useful to me. However, an iterating >> version of it doesn't sound any more useful. What would it >> gain you? Why not just iterate over the set? Or make a >> copy and repeatedly pop() it? > > Since I was the person who decided that "arbitrary" meant "give a > different result each time", I should answer that. > > My intention was not that people should use repeated calls to pick() for > iteration. I described that as an abuse of the method. But I can > imagine scenarios where the caller calls set.pick() sequentially: > > def pick_two_cards(hand): > assert isinstance(hand, (set, frozenset)) > assert len(hand) == 5 > return (hand.pick(), hand.pick()) > > > As I said in a reply to Raymond, optimization was not my primary > concern, but as a fundamental set operation[1] pick() should be > efficient. It may be called on a set with millions of items, not just > small sets. Copying to another set, or to a list, will be O(N) and > potentially slow -- at the very least, it is wasteful to copy millions > of elements in order to access one. > > I don't know how expensive it is to create a set iterator, but (my > reasoning went) surely we can do better? The set already has access to > its own elements, it just needs to return one of them. pop() also knows > how to retrieve an arbitrary element. pick() is just like pop(), except > without removal. > > >> I completely fail to see a use case for this. > > Nevertheless, people keep requesting it, so obviously they have a use > for it. None of the standard solutions are obvious or easily > discoverable, and in my experience the usual solution people come up > with is to pop() an element, then add() it back in, but of course > that's not just inelegant but it fails on frozensets. > > > > [1] Obviously there is disagreement on whether or not pick() is a > fundamental operation or not. As Raymond points out, it is uncommon in > other languages. But Wikipedia lists it as fundamental, and it is not > just Python users who ask for it: > > http://stackoverflow.com/questions/124671/picking-a-random-element-from-a-set
You're obviously talking about a *random* element. This is a separate use case (though I agree many people don't know the difference). Picking a random element can be done in O(1) only if the data structure supports access by index, which Python's hash tables don't. -- --Guido van Rossum (python.org/~guido) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
